The present invention concerns an automatic safety belt system of the kind that includes a belt band wound on a spring-loaded roller and a device for signaling when a length of the belt sufficient for application about a user's body has been unwound.
A method is known (e.g. from German Letters of Disclosure No. 20 06 029 and U.S. Pat. No. 3,147,819) whereby an electric switch is arranged in a belt lock used to fasten the belt about a user's body, which switch prevents the engine from starting when the belt has not been locked (a so-called "interlock" system) or transmits a signal to the driver indicating that the safety belts have not been applied.
It is known that a signal indicating the proper application of the safety belt can be generated by means of a device arranged on or in the automatic-belt-retractor housing, which device determines how much of the belt band has been unwound (e.g. German Letters of Disclosure No. 22 17 443 and U.S. Pat. No. 3,506,305). In a known arrangement (German Letters of Disclosure No. 22 17 443), this device is designed as an electric switch. The belt band, on being pulled out of the housing, is pulled through the case of the electric switch and holds a movable switch contact arranged with the case in one position. The movable contact is allowed to move to a new position when the belt is unwound because of a perforation (hole, etc.) provided in the belt band. Thus, as soon as the belt band has been unwound a sufficient length from the reel, movement of the switch contact causes a corresponding modification of the switching state thereof. Perforations and the like arranged in the belt constitute problems in that the cross section of the belt band is weakened. Therefore, it may become necessary to increase the belt dimensions. Moreover, the functional efficiency of the safety belt over the long run may be put in doubt in that the mechanical contact slides along the belt band which, by necessity, results in a certain wear of the belt.
In another device, known from U.S. Pat. No. 3,506,305, the decrease in the diameter of the coiled belt when it is pulled off the reel is determined and is utilized as a signal indicating the proper application of the safety belt. Such a diameter examination can be relatively imprecise in that the correlation between the decrease in diameter and the unwound belt length is not very great.